#include<iostream>
#include<cstring>

const char cstr1[] = "Hello";
const char cstr2[] = "world!";

int main()
{
	constexpr size_t new_size = strlen(cstr1) + strlen(" ") + strlen(cstr2) + 1;
	char cstr3[new_size];
	
	strcpy(cstr3,cstr1);
	strcat(cstr3," ");
	strcat(cstr3,cstr2);
	
	std::cout << cstr3 << std::endl;
}
